The overall goal of this grant is to identify critical genes and biochemical pathways involved in normal megakaryocyte (MK) differentiation. We propose to do this by studying rare clinical conditions in which MK maturation and platelet production are abnormal, resulting in inherited thrombocytopenia or thrombocytosis. Concurrently, a "systems biology" approach will be applied to study and interpret the molecular fingerprint of abnormal MKs and platelets from these pedigrees in comparison to normal individuals. Specific Aims for this proposal are: Aim 1: Evaluate and study novel forms of inherited/congenital thrombocytopenia and thrombocytosis. Aim 1A: Confirm and characterize a gone for autosomal dominant thrombocytopenia on chromosome 10. Having previously limited the linkage interval to approximately 5 x 10 6 bases of genomic DNA, we will identify the mutation within this interval and study the normal function of the encoded gone product. Aim 1B: Study rare families with previously uncharacterized forms of inherited thrombocytopenia or thrombocytosis and use genetic techniques to localize the genetic mutation. Aim 2: Examine differences in the transcriptome and proteome of primary cells isolated from normal adults and individuals with inherited thrombocytopenia and thrombocythemia in order to recognize and understand the downstream effects of specific mutations. Successful completion of these Aims will provide benefits to scientists, hematologists, and patients. First, it may identify novel signaling pathways, transcription factors, or cell cycle regulators that control megakaryocytopoiesis. These cellular mechanisms will help explain the specialized processes that result in polyploidy and programmed cytoplasmic fragmentation (i.e. thrombopoiesis). Clinically, these studies will result in greater awareness of inherited thrombocytopenia, potentially leading to earlier identification, better therapies, and avoiding unnecessary procedures when an incorrect diagnosis is made. Finally, these studies may help in the targeted development of better platelet lowering drugs using rational design to improve on currently available options such as anagrelide, hydroxyurea, and interferon.